The Lebanon-based Hezbollah organization, one of the world’s most formidable terrorist groups, is increasingly teaming up with al-Qaida on logistics and training for terrorist operations, according to U.S. and European intelligence officials and terrorism experts.

The new cooperation, which is ad hoc and tactical and involves mid- and low-level operatives, mutes years of rivalry between Hezbollah, which draws its support primarily from Shiite Muslims, and al-Qaida, which is predominantly Sunni. It includes coordination on explosives and tactics training, money laundering, weapons smuggling and acquiring forged documents, according to knowledgeable sources.

This new alliance, even if informal, has greatly concerned U.S. officials in Washington and intelligence operatives abroad who believe the assets and organization of Hezbollah’s formidable militant wing will enable a hobbled al-Qaida network to increase its ability to launch attacks against American targets.

The concerns over the new partnership have reached the Senate and House intelligence committees’ chairmen and vice chairmen, who, under special rules, are regularly briefed by CIA Director George Tenet and FBI Director Robert Mueller on highly classified information and operations not revealed to other committee members.

The new collaboration illustrates what analysts say is an evolving pattern of decentralized alliances between terrorist groups and cells that share enough goals to find common ground: crippling the United States, and forcing the U.S. military out of the Middle East and Israel out of Palestinian territory.

“There’s a convergence of objectives,” said Steven Simon, a former National Security Council terrorism expert. “There’s something in the zeitgeist that is pretty well-established now.”

Although cooperation between al-Qaida and Hezbollah may have been going on at some level for years, the U.S. war against al-Qaida has hastened and deepened the relationship. U.S. officials believe that after al-Qaida was driven from Afghanistan, leader Osama bin Laden sanctioned his operatives to ally themselves with helpful Islamic-based groups, said a senior administration official with access to daily intelligence reports.

Bin Laden or his top associates have used the Internet to convey this message, the official added. There is “no doubt at all” that Hezbollah and al-Qaida have communicated on logistical matters, the official said.

Loose partnerships are being facilitated by members’ ability to communicate using Internet chat rooms accessible with constantly changing passwords. The connections, intelligence officials believe, are made case by case, depending on the needs of a particular local group. “When someone’s traveling and needs assistance in passing through, whomever happens to have that capacity will be turned to,” said Paul Pillar, former deputy director of the CIA counterterrorism center and author of “Terrorism and U.S. Foreign Policy.”

The chat rooms are set up to avoid detection. New recruits can enter only rooms where “holy war” against America or other general topics are discussed. Only trusted and vetted operatives can access chat rooms where specific deals are discussed.

Hezbollah’s original goal was to create an Islamic state in Lebanon. For 18 years, with financial and intelligence support from Iran and Syria, the group fought to end Israel’s military occupation of a buffer zone in southern Lebanon. It attacked American targets in a bid to drive the United States from the country.

There’s little dispute that al-Qaida and Hezbollah operatives work together, but some analysts reject the notion that the two groups have buried their differences, which have long been sharp because they derive their support from the two competing branches of Islam. “I just don’t see it,” said Kenneth Katzman, a Middle East specialist at the Congressional Research Service. “There’s not a lot of commonality there.”

Although all of Hezbollah’s attacks have taken place overseas, the FBI is investigating close to a dozen Hezbollah groups in the United States. Their ostensible purpose is to raise money for Lebanese charities.

But a recent criminal court case in Charlotte in which eight defendants pleaded guilty and two were later found guilty by a jury showed how what prosecutors alleged was one Hezbollah cell involved in cigarette smuggling conspired to aid the organization as a whole. One of the men, Mohamad Hammoud, was caught on wiretaps speaking on the phone with Hezbollah’s military commander in Lebanon, Sheik Abbas Harake.

Court documents in the United States and Canada say Hezbollah members in both countries have tried to procure military equipment, including laser-range finders, aircraft-analysis software, global positioning gear, night-vision goggles, blasting equipment and mine detection machinery for fighters in Lebanon.

U.S. law enforcement officials and terrorism experts fear the infrastructure and personal relationships established to facilitate illicit arms and document purchases could easily be used to launch attacks on U.S. soil.

“It gives you an infrastructure you can potentially build on,” Pillar said. That is what analysts believe happened in Argentina in 1996, when Hezbollah, which had longtime financial and logistics networks in Paraguay, Brazil and Argentina, bombed the Israeli Embassy in Buenos Aires.

In the last 18 months, Hezbollah has reactivated some of its overseas assets in South America, Europe and Central Asia, Simon said. “They appear to be cocking their guns again.”

The more recent relationship between Hezbollah and al-Qaida first surfaced publicly in testimony in October 2000 by Ali Mohamed, a former U.S. Green Beret who pleaded guilty to conspiring with bin Laden to bomb U.S. embassies in Africa.

He testified to having provided security for a meeting in Sudan “between al-Qaida … and Iran and Hezbollah … between Mughniyah, Hezbollah’s chief, and bin Laden.” Hezbollah, he testified, provided explosives training to al-Qaida while Iran “used Hezbollah to supply explosives that were disguised to look like rocks.”